Method for determining a force at the hub of a wheel of a vehicle while traveling and wheel suitable for allowing said method to be carried out

ABSTRACT

A method for determining the force at the hub of a wheel of a vehicle while traveling. The wheel includes a rim and at least one deformation sensor directly associated with the rim in at least one predetermined position and arranged according to at least one predetermined orientation. The method includes the steps of detecting at least one deformation component of the rim during travel through the at least one deformation sensor; applying to the at least one deformation component, during travel, a correlation parameter characteristic of the rim, between the force at the hub and the relative deformation of the rim to determine at least one force component at the hub correlated with the at least one deformation component of the rim. The determining of the correlation parameter preferably takes place through experimental tests that include the steps of providing a sample wheel having a rim substantially the same as that of the wheel and determining at least one correlation coefficient between at least one force component acting upon the sample wheel at the hub along at least one predetermined direction and at least one respective deformation component of the rim of the wheel.

The present invention relates to a method for determining a force at thehub of a wheel of a vehicle whilst traveling. More specifically, theinvention relates to a method for measuring the forces at the hubgenerated by the wheel-road contact forces acting on the pneumatic tyreof a vehicle wheel whilst traveling. The invention also relates to awheel for vehicles suitable for allowing said method to be carried out.

Throughout the present description and in the subsequent claims, theexpression: wheel-road contact forces, is used to indicate the stressesapplied on the wheel by the road surface (or by a device which simulatesthe road surface, such as a rolling runway of a drum simulating the roadsurface). Such stresses include, for example, the vertical and/orlongitudinal and/or lateral forces acting locally on the pneumatic tyreand/or the moments generated by such forces.

As known, in the field of the vehicle dynamic control, a substantialpart of the research has as a primary purpose that of allowing the realtime monitoring of the behavior of wheels whilst they are traveling.This is in order to contribute to ensuring dynamic control of thevehicle in the different conditions of use, for such a purpose suitablyadjusting possible safety and/or comfort devices foreseen on thevehicle, such as ABS (Anti Block System), VDC (Vehicle Dynamic Control),suspension adjustment devices, steering adjustment devices, poweradjustment devices and transmission adjustment devices. The real timemonitoring of the wheels can also be useful for identifying the causesof accidents or other diagnostic and/or precautionary purposes.

For such a purpose, wheels and vehicles can be equipped with devicescapable of detecting in real time information relative to the currentdynamic conditions of the wheel whilst traveling. Such information isused on the same vehicle to suitably adjust the aforementioned safetyand/or comfort devices so as to contribute to ensuring that the driverhas the control of the vehicle in those specific conditions of use. Theaforementioned information can also be stored or sent telematically to aremote control station to carry out a diagnostic analysis of the wheel.

Amongst the information considered of interest for achieving theaforementioned purposes there are those linked to the magnitude anddistribution of the stresses applied by the road surface on thepneumatic tyre and the deformations of the wheels under a load.

For example, the information relative to the stresses acting on eachpneumatic tyre and transmitted to the hub can be used to suitably adjustthe braking force on each pneumatic tyre (for example through ABS). Suchinformation can also be used to determine the dynamic stability of thevehicle and thus suitably adjusting the steering, when it is used tocontrol the lateral dynamics. Moreover, such information can also beused to adjust the rigidity and behavior of the suspension system actingon each pneumatic tyre.

Devices and methods for the real time monitoring of the wheels of avehicle through detection of the deformations of the rim due to stressesapplied by the road surface to the pneumatic tyre are known.

For example, US 2003/0058118 A1 discloses a system for monitoring apneumatic tyre of a vehicle, in which the deformation of a pneumatictyre under a load is detected to obtain information linked to such adeformation, such as the load on the pneumatic tyre, the total mass ofthe vehicle and the mass distribution of the vehicle. The deformation ofthe pneumatic tyre is detected through detection of the variations inacceleration of the pneumatic tyre in rotation (by means of anacceleration sensor mounted on the pneumatic tyre, preferably on aninner surface of the tread) and generation of electric signalsrepresenting the variations in acceleration at the wheel-road contactzone. The Applicant has noted that the actuation of the method describedin such a document requires the arrangement of an acceleration sensorinside the pneumatic tyre. Therefore, this is a detection system that isinvasive on the pneumatic tyre and that thus necessarily provides usefulresults only for that specific pneumatic tyre on which the accelerationsensor is installed. Such a system also involves technical complicationsin the assembly of the wheel, whereas the addition of mass to thepneumatic tyre implies greater problems of uniformity, or else lesscomfort associated with irregular and greater wear.

Different devices and methods are known that foresee a measurement ofthe wheel-road contact forces through the use of strain gauges mountedon the wheel in positions other than the pneumatic tyre.

For example WO 03/008243 discloses a device for detecting the forcesacting on a pneumatic tyre, such a device comprising a disc intended tobe mounted between the hub and the rim of the wheel and provided with aplurality of strain gauges suitable for detecting the forces acting onthe pneumatic tyre, and a detection portion suitable for determining anabnormality in the detection of the forces acting on the pneumatic tyreto allow a subsequent correction of such an abnormality.

U.S. Pat. No. 6,681,646 discloses a device for detecting thedeformations of a wheel for a vehicle comprising a substantially annularelement placed between the hub and the rim of the wheel and on which aplurality of strain gauges suitable for detecting the forces acting uponthe pneumatic tyre are mounted.

The Applicant has noted that the devices described in the two prior artdocuments discussed above require the arrangement in the wheel ofadditional rotating masses on which the strain gauges are to be mounted.From this derives a technical complication in the assembly operations ofthe wheel, a significant increase in the overall weight of the wheel andan alteration of the dynamic behavior of the wheel itself.

The Applicant has considered the problem of determining in real time theforces at the hub of a wheel of a vehicle generated by the wheel-roadcontact stresses acting whilst traveling through a system for detectingdeformations which, as well as allowing, through suitable algorithms, areliable measurement of the actual stresses/forces at the hub generatedby the contact between pneumatic tyre and road surface in the differentconditions of use to be obtained, is non-invasive on the pneumatic tyre(so as to make the method able to be carried out irrespective of thespecific type of pneumatic tyre mounted on the rim) and does not involvea significant increase in the overall weight of the wheel nor analteration of the dynamic behavior of the wheel itself. The Applicanthas thus verified that such a problem is solved by associating at leastone deformation sensor directly on the rim of the wheel in at least onepredetermined position and according to at least one predeterminedorientation, so as to be able to detect, during travel, at least onedeformation component of the rim of the wheel and then apply to said atleast one deformation component a correlation parameter characteristicof that specific rim.

The present invention therefore relates, in a first aspect thereof, to amethod for determining a force at the hub of a wheel of a vehicle whilsttraveling, said wheel comprising a rim, a pneumatic tyre mounted on saidrim and at least one deformation sensor directly associated with saidrim in at least one predetermined position and arranged according to atleast one predetermined orientation, the method comprising the steps of:

-   -   detecting during travel at least one deformation component of        said rim through said at least one deformation sensor;    -   applying to said at least one deformation component, during        travel, a correlation parameter, characteristic of said rim,        between the force at the hub and the relative deformation of        said rim to determine at least one force component at the hub        correlated with said at least one deformation component of said        rim.

Throughout the present description and in the subsequent claims, theexpression: deformation sensor, is used to indicate any device capableof detecting any characteristic magnitude that is representative of adeformation of the rim on which the sensor is mounted. For example,detection devices suitable for carrying out the method of the presentinvention are: strain gauges, accelerometer, membrane sensors (forexample sensors based upon SAW technology), optical sensors, pressuresensors, sensors that vary their magnetic properties as the stresses towhich they are subjected vary, and the like.

The expression: relative deformations, or respective deformations, onthe other hand, is used in the rest of the present description and inthe subsequent claims to indicate specific deformations, or specificdeformation components, caused by specific forces, or by specific forcecomponents.

The method of the present invention therefore advantageously requiresthe arrangement of a wheel comprising at least one deformation sensordirectly associated with the rim of the wheel (for example housed insuitable recesses formed on the rim or embedded in the rim at themelting step or housed in suitable manufactured products in turnincluded in the rim). In other words, it is a wheel provided with asystem for detecting the deformations that, unlike the systems discussedabove with reference to the prior art, is not invasive on the pneumatictyre. Since the system for detecting the deformations of the wheel ofthe present invention does not require any significant rotating massadded to the wheel, it does not involve an increase in the overallweight of the wheel, nor an alteration of the dynamic behavior of thewheel itself. It follows that a simplification of the assemblyoperations of the wheel is achieved.

The correlation parameter used in the method of the present invention isa force-deformation correlation parameter characteristic of thatspecific rim of the wheel of which one wishes to known the forces at thehub generated during travel by the wheel-road contact forces. Therefore,the application of such a correlation parameter to the deformationsdetected in real time on the rim of the wheel during travel allows theforces at the hub generated by the wheel-road contact forces actingduring travel on the pneumatic tyre and effectively responsible forthose deformations detected through the sensors to be determined.Assuming that the relation between deformations of the rim and forces atthe hub is linear, it follows that:

F=[A]·ε

where F is the force at the hub, [A] is the correlation parameter and εis the deformation caused by the force F. The application of thecorrelation parameter [A] to the deformations ε thus allows the forces Fto be obtained.

The position, the orientation and the number of deformation sensors onthe rim can vary according to the specific requirements and/or needs. Inparticular, in the case in which one wished to measure just one specificforce component at the hub, for example just the longitudinal forcecomponent Fx, or just the lateral force component Fy or just thevertical force component Fz, it is sufficient to apply just one straingauge onto the rim in an appropriate position and according to anappropriate orientation; the correlation parameter [A], in such a case,shall assume the form of a vector. On the other hand, in the case inwhich one wished to measure more than one force component at the hub,for example the three components Fx, Fy and Fz defined above, it isnecessary to apply at least three strain gauges to the rim; in such acase F and ε shall be vectors and the correlation parameter [A] shallassume the form of a matrix.

Preferably, the correlation parameter is obtained through the executionof experimental tests on the wheel itself (or, preferably on a wheelanalogous to that) for which one wishes to know the forces at the hubduring travel. Such experimental tests consist of simulating in alaboratory the stresses applied by the road surface on the pneumatictyre of the wheel during travel (as at least one test parameter isvaried, such as the drift angle, camber angle, vertical force applied,pneumatic tyre pressure, slidding of the pneumatic tyre (when braking),speed of the wheel) and detecting the respective deformations of the rimof the wheel to then determine the force-deformation correlationparameter characteristic of that specific rim on which the experimentaltests have been carried out.

More preferably, the determination of the correlation comprises thesteps of:

-   a) providing a sample wheel having a rim which is substantially the    same as the rim of said wheel;-   b) determining, through experimental tests on said sample wheel, at    least one correlation coefficient between at least one force    component acting upon said sample wheel at the hub along at least    one predetermined direction and at least one respective deformation    component of the rim of said sample wheel.

Throughout the present invention and in the subsequent claims, theexpression: sample wheel, is used to indicate a wheel provided with arim identical to the rim of the wheel for which one wishes to measurethe forces at the hub during travel, equipped with any pneumatic tyre,and having the same type, equal number and analogous arrangement andorientation of sensors with respect to the aforementioned wheel.

Preferably, the step of determining said at least one correlationcoefficient comprises the steps of:

-   a1) setting said correlation coefficient at an initial value;-   a2) applying at least one test parameter to said sample wheel;-   a3) applying at least one force component acting upon the sample    wheel at the hub along said at least one predetermined direction;-   a4) detecting at least one deformation component of the rim of said    sample wheel, caused by said at least one force component;-   a5) multiplying said at least one correlation coefficient and said    at least one deformation component detected in step a4) to obtain at    least one calculated force component;-   a6) comparing said at least one force component calculated in step    a5) with said at least one force component applied in step a3);-   a7) iteratively repeating steps a5) and a6) each time correcting the    value of said at least one correlation coefficient until said at    least one force component calculated in step a5) assumes a value    substantially equal, apart from a predetermined tolerance, to that    of said at least one force component applied in step a3).

The correlation parameter is thus obtained by carrying out experimentaltests on a conventional testing device provided with a suitable equippedhub on which the sample wheel is to be mounted, with a rolling runwaysuitable for simulating the road surface (for example a flat belt), witha stressing member suitable for applying a predetermined force to thesample wheel according to a predetermined direction and with a devicefor measuring the force components exchanged, following the applicationof the load, between hub and sample wheel that rolls on said flat belt(hereafter such forces are also indicated as forces at the hub). Thecomparison between forces determined at the hub and forces applied atthe hub and the continuous correction of the correlation parameter untilthe forces determined at the hub are equal, apart from a predeterminedtolerance, to the forces applied at the hub then allows identification,by a optimisation process, of that particular correlation parameterbetween forces at the hub and respective deformations which ischaracteristic of that particular rim.

Preferably, in a preferred embodiment of the method of the presentinvention, the correction of said at least one correlation coefficienttakes place through an error minimization algorithm between said atleast one force component calculated in step a5) and said at least oneforce component applied in step a3). However, it is possible to useother conventional calculation algorithms.

Preferably, said at least one deformation sensor is directly associatedwith the rim of said wheel at at least one zone of maximum deformationof said rim in a determined direction. The optimal points of the rim arethus identified on which the deformation sensors are to be positioned invarious ways in order to make easier and optimise the detection of thedeformations of the rim. The zone of maximum concentration ofdeformation, for example, can be identified through a finite elementsanalysis of the rim itself.

Preferably, said at least one deformation sensor is a strain gauge. Sucha sensor is, indeed, known to allow an efficient and effective detectionof the local deformations of any metal body.

In a variant of the method of the present invention, said at least onedeformation sensor is a membrane sensor. For example, said at least onedeformation sensor is a membrane sensor preferably based upon SAWtechnology. This are sensors having a rigidity, and consequently aresonance frequency, which varies as the stresses to which they aresubjected varies. Such sensors, although more sophisticated and delicatethan strain gauges, have the advantage with respect to the latter of notrequiring the presence of an electric power source on the wheel.

It is also possible to use sensors based upon materials that have avariation in their magnetic properties as the stress to which they aresubjected varies. The variation in magnetic properties is usuallymeasured through a variation in inductance.

However, in the actuation of the method of the present invention it ispossible to use other conventional sensors, such as accelerometers,magnetic sensors and optical sensors, provided that they are able toallow the acquisition of information representing the local deformationsof the rim of the wheel under a load during the rolling of the wheelitself.

Preferably, said wheel comprises at least three deformation sensorssuitably positioned in different points of the wheel so as to detect atleast three local deformation components of the rim generated by theforce acting at the wheel-road contact zone.

In the preferred embodiment of the method of the present invention, thedeformation sensors associated with the wheel are four in number, so asto always ensure the presence of at least three active sensors even inthe case in which one fails.

Preferably, the method of the present invention also comprises the stepsof:

-   -   generating at least one control signal representing said at        least one force component determined at the hub during travel;    -   processing said at least one control signal to determine data        representing the dynamic conditions of the wheel during travel;    -   sending said data to a vehicle control unit.

Advantageously, the data representing the dynamic conditions of thewheel during travel is used on the same vehicle to suitably adjust theaforementioned safety and/or comfort devices so as to contribute toensuring that the driver has the control of the vehicle in the differentconditions of use. The aforementioned data can also be stored or senttelematically to a remote control station to carry out a diagnosticanalysis of the wheel.

The processing step of said at least one control signal is preferablycarried out by a signal processing unit mounted on said wheel or,according to a variant of the method of the present invention, insidethe vehicle.

The control unit can be mounted inside the vehicle or in a remoteposition with respect to the vehicle. For example, the control unit isprovided inside the vehicle when one wishes to carry out the adjustmentof the possible safety and/or comfort devices associated with the wheel,whereas it can be provided in a remote position with respect to thevehicle when one wishes to store the data to subsequently carry out adiagnostic analysis of the wheel.

In the case in which one wishes to contribute to the adjustment of thepossible safety and/or comfort devices associated with the wheel, themethod of the present invention comprises the steps of:

-   -   processing said data in said vehicle control unit to generate a        command signal representing optimal dynamic conditions of said        wheel during travel;    -   controlling the dynamic behavior of the vehicle modifying, based        upon said command signal, at least one travel parameter applied        to said wheel. Such a travel parameter is, for example, the        drift angle, the camber angle, the vertical force applied, the        pneumatic tyre pressure, the sliding of the pneumatic tyre (when        braking), the speed of the wheel.

The actuation of the method of the present invention requires thepresence of a power source for the operation of the deformation sensors(in the case of strain gauges) and for the data processing andtransmission. Such a power source, per sé conventional, can be mountedinside the wheel (in the case in which the wheel comprises possibledevices requiring a power supply, like strain gauges and processingunits of the control signal) or, alternatively, inside the vehicle (inthe case in which deformation sensors are used that do not require apower supply and the processing unit of the control signal is foreseeninside the vehicle). In the latter case, the transmission of the powersupply to the wheel takes place by radio frequency.

In a second aspect thereof, the present invention relates to a wheel forvehicles comprising a rim and a pneumatic tyre mounted on said rim, inwhich said wheel comprises at least one deformation sensor directlyassociated with said rim in at least one predetermined position andarranged according to at least one predetermined orientation.

Such a wheel allows the method of the present invention discussed aboveto be carried out and thus has, at least in part, the same advantageouscharacteristics highlighted above with reference to such a method. Inparticular, with such a wheel the drawbacks encountered in the wheels ofthe prior art previously discussed (invasiveness on the pneumatic tyre,significant increase in overall weight, alteration of the dynamicconditions and complications in the assembly operations) are overcome.

Preferably, said at least one deformation sensor is associated with saidrim at at least one zone of maximum deformation of said rim in adetermined direction.

In a first preferred embodiment of the wheel of the present invention,said at least one deformation sensor is a strain gauge. In a variantthereof, said at least one deformation sensor is a membrane sensorhaving a rigidity, and consequently a resonance frequency, which variesas the stresses to which the membrane is subjected vary.

In the preferred embodiment of the wheel of the present invention, thewheel comprises at least three deformation sensors suitably positionedin different points of the wheel so as to detect at least three localdeformation components of the wheel generated by the force acting at thewheel-road contact zone. Even more preferably, the wheel comprises foursensors.

In an embodiment thereof, the wheel of the present invention alsocomprises a processing unit of a control signal representing at leastone force component at the hub.

Preferably, the wheel of the present invention also comprises a powersource.

Further characteristics and advantages of the present invention shallbecome clearer from the following detailed description of someembodiments thereof, made with reference to the attached drawings. Insuch drawings:

FIG. 1 shows a block diagram of the method of the present invention inits most general embodiment;

FIG. 2 shows a schematic perspective view of an embodiment of a samplewheel used in a preferred embodiment of the method of the presentinvention;

FIG. 3 shows an example diagram of a first embodiment of the method ofthe present invention;

FIG. 4 shows an example diagram of a second embodiment of the method ofthe present invention;

FIG. 5 shows an example diagram of an embodiment of the method of thepresent invention for controlling the dynamic behavior of the wheel;

FIG. 6 shows the results obtained by the Applicant in an experimentaltest on the sample wheel of FIG. 2 with variation of vertical load from2000 N to 6000 N at 50 km/h, and zero drift, sliding and camber;

FIG. 7 shows the results obtained by the Applicant in an experimentaltest on the sample wheel of FIG. 2 with sinusoidal variation of drift(magnitude 2°, frequency 0.25 Hz) at 50 Km/h, 3000 and 5000 N verticalload and zero sliding and camber;

FIG. 8 shows the results obtained by the Applicant in an experimentaltest on the sample wheel of FIG. 2 with sinusoidal variation of camber(magnitude 2°, frequency 0.25 Hz) at 50 Km/h, 3000 and 5000 N verticalload and zero sliding and drift;

FIG. 9 shows the results obtained by the Applicant in an experimentaltest on the sample wheel of FIG. 2 with 0-10-0% sliding braking at 50km/h, 3000 and 5000 N vertical load and zero drift and camber.

In FIG. 1 the main steps of an embodiment of a method for measuring theforces at the hub in a wheel of a vehicle whilst traveling, inaccordance with the present invention, are shown.

The actuation of such a method firstly comprises a step of arranging,directly on the rim of the wheel and in predetermined positions, aplurality of deformation sensors suitably orientated so as to be able todetect a plurality of deformation components during travel. Preferably,the aforementioned predetermined positions are identified in an optimalmariner by using simulation techniques, for example the technique offinite elements analysis.

In accordance with the method of the present invention, the forces atthe hub generated by the wheel-road contact forces acting on thepneumatic tyre of the wheel during travel are determined by detectingthe deformations during travel of the rim through the aforementioneddeformation sensors and applying to said deformations an appropriatecorrelation parameter between forces at the hub and respectivedeformations (see FIG. 1).

The correlation parameter is a characteristic parameter of the rim inquestion and the application of such a parameter to the deformationsdetected during travel on that rim allows the determination of the forcecomponents at the hub actually present and generated by the wheel-roadcontact forces acting on the wheel and responsible for thosedeformations detected by the sensors. For such a purpose it can beassumed that the relation between deformations of the rim and forces atthe hub is linear, that is:

F=[A]·ε

where F is the force at the hub, [A] is the correlation parameter and εis the deformation caused by the force F. The application of thecorrelation parameter [A] to the deformations ε thus allows the forces Fto be obtained.

Hereafter, explicit reference shall be made to the case in which onewishes to measure three perpendicular components Fx, Fy and Fz of theforce at the hub, where Fx is the longitudinal force component (that isthe force component along the rolling direction of the pneumatic tyre),Fy is the lateral force component (that is the force component along theaxis of the hub of the wheel) and Fz is the vertical force component. Insuch a case, the rim of the wheel is equipped with at least threedeformation sensors; in such a case F and ε shall be vectors and thecorrelation parameter [A] shall assume the form of a 3×3 matrix formedof 9 correlation coefficients.

In the preferred embodiment of the wheel of the present invention, thereare four sensors. The sensors are also associated with the rim atrespective zones of maximum deformation of the rim along directions x, yand z. Such zones of maximum concentration of deformation areidentified, for example, by carrying out a finite elements analysis onthe rim, as stated previously.

The correlation matrix is obtained through the execution of experimentaltests on a sample wheel, that is on a wheel provided with a rimidentical to that of the wheel for which one wishes to measure theforces at the hub during travel, and having the same type, equal numberand analogous arrangement and orientation of sensors with respect to theaforementioned wheel. To determine the various coefficients a series oftests are carried out in which various test parameters are varied inturn, or simultaneously, for example drift angle, camber angle, verticalforce applied, pneumatic tyre pressure, sliding of the pneumatic tyre(when braking), speed of the wheel.

FIG. 2 shows a schematic example embodiment of a sample wheel,generically indicated with 1, and of a device 100, per sé conventional,for carrying out the aforementioned experimental tests.

The sample wheel 1 of FIG. 2 comprises a rim 2 on which a pneumatic tyre3 is mounted. The rim 2, in particular, has a plurality of spokes,indicated with 4 a, 4 b, 4 c, 4 d and 4 e, a central annular portion 5for attachment to a hub and a peripheral annular portion 6 forattachment to the pneumatic tyre 3. Of course, the specific shape of therim, the presence or not of spokes and possibly the number of suchspokes has absolutely no influence for the purposes of the actuation ofthe method of the present invention; what is important is that the rim 2of the sample wheel 1 on which the experimental tests are carried out isidentical (also taking into account the presence of the deformationsensors) to that of the wheel for which one wishes to measure the forcesat the hub during travel.

The position, the number and the orientation of the deformation sensorson the rim 2 can vary according to the specific requirements. Theexample sample wheel illustrated in FIG. 2 comprises three deformationsensors directly associated with the rim 2; such a wheel allows, forexample, the detection of three deformation components of the rimassociated with the three perpendicular components Fx, Fy and Fzidentified above.

In the sample wheel illustrated in FIG. 2, the sensors are positioned inthe following way:

-   -   a first sensor 71 is mounted on the front surface of the spoke 4        a in a position and with an orientation such as to allow the        detection of the deformation of the rim 2 by flexing in the        plane perpendicular to the middle plane X-Z of the wheel 1 (such        a deformation is substantially caused by the combined action of        the force components Fy and Fz);    -   a second sensor 72 is mounted on the annular portion 5 for        attachment to the hub at the spoke 4 b in a position and with an        orientation such as to allow the detection of the        circumferential deformation of the rim 2 (such a deformation is        substantially caused by the action of the force component Fy);    -   a third sensor 73 is mounted on a side rib 40 of the spoke 4 b        in a position and with an orientation such as to allow the        detection of a deformation of the rim 2 by flexing in a plane        parallel to the middle plane X-Z of the wheel 1 (such a        deformation is substantially caused by the action of the force        component Fx).

As illustrated in FIG. 2, the device 100 for carrying out theexperimental tests for determining the correlation matrix comprises ahub 101 on which the sample wheel 1 is mounted and a member 102 actingon the hub 101 to apply a predetermined force to the sample wheel 1,through the hub 101, along a predetermined direction. The test device100 also comprises a rolling runway 103 suitable for simulating the roadsurface and a force measuring device (not illustrated) suitable formeasuring the components of the force applied (previously applied to thehub of the sample wheel 1).

The determining of the coefficients of the correlation matrix takesplace in the following way: once the sample wheel 1 has been mounted onthe hub 101 and at least one test parameter to be applied to the wheelhas been selected (for example drift angle, camber angle, vertical forceapplied, pneumatic tyre pressure, sliding of the pneumatic tyre, speedof the wheel) the wheel is stressed with a predetermined force at thehub. The force components Fx, Fy and Fz acting upon the hub of thesample wheel 1 along the three perpendicular directions x-x, y-y and z-zfollowing the application of the predetermined force are measuredthrough the force measuring device, whereas the three deformationcomponents caused by said three force components are measured throughthe sensors 71, 72 and 73. Then the coefficients of the correlationmatrix are initially set at predetermined random values and three forcecomponents are calculated by applying the correlation matrix to thedeformation components detected by the sensors. Then these calculatedforce components are compared with the force components applied at thehub and measured by the force measuring device and the aforementionedsteps of calculation of the force components and of comparing thesecalculated components with those applied and measured by the forcemeasuring device, each time correcting the value of the coefficients ofthe correlation matrix through an appropriate algorithm. The continuousprocess is interrupted as soon as the values of the calculated forcecomponents equal, apart from a predetermined tolerance, the values ofthe force components applied to the hub.

The correction of the coefficients of the correlation matrix preferablytakes place through an error minimization algorithm between thecalculated force components and the force components applied at the hub(and measured).

The correlation matrix between forces at the hub and respectivedeformations is thus determined experimentally. Such a matrix allows,once the deformation components detected by the sensors of the wheelduring travel are known, the force components at the hub, that aresubstantially equal, apart from the predetermined tolerance, to thecomponents at the hub actually acting on the wheel, to be determined.

In practice, the correlation parameter, that is in the case illustratedabove the matrix [A], through its own coefficients, decouples the forcesacting upon the hub during travel according to the aforementioned threeaxes.

In the preferred embodiment of the method of the present invention, oncethe forces at the hub acting upon the wheel during travel have beencalculated, a control signal representing said forces is generated. Sucha signal is sampled and then sent to a suitable processing unit where itis processed to determine the data representing the dynamic conditions(forces at the hub in the three directions) of the wheel during travel.The processing unit of the signal can be mounted inside the wheel (asillustrated in FIG. 3) or inside the vehicle (as illustrated in FIG. 4).In the first case the wheel shall be provided with a suitable datatransmission antenna (as well as an appropriate electrical power supplysystem, if necessary) and the vehicle shall be provided with a suitabledata receiving antenna.

The data processed in the processing unit is then sent to a control unitof the dynamics of the vehicle (ABS, VDC, etc.), which uses the datareceived, as well as other data coming from the vehicle itself (forexample yaw acceleration, etc.) to calculate the suitable and possiblevariations on the actuator members (for example brakes, suspension unit)in order to optimise the dynamic behavior of the vehicle (FIG. 5). Thecontrol unit can be mounted inside the vehicle or alternatively in aremote position with respect to the vehicle.

The data processed by the control unit can also be stored or senttelematically to a remote control station to carry out a diagnosticand/or statistical analysis of the wheel.

The actuation of the method of the present invention requires thepresence of an electric power source for the deformation sensors (in thecase of strain gauges) and for the processing and transmission of data.Such a power source, per sé conventional, can be mounted inside thewheel (in the case in which the wheel comprises possible devices thatrequire an electric power supply, like strain gauges and processing unitof the control signal) or, alternatively, inside the vehicle (in thecase in which deformation sensors are used that do not require anelectric power supply and the processing unit of the control signal isprovided inside the vehicle).

In the specific example illustrated in FIG. 2, the sensors used arestrain gauges. They effectively measure the local deformations of therim.

For the method of the present invention to be carried out correctly theinterfering effects of the temperature upon the strain gauges must betaken into account.

As is known, the main effect of temperature upon strain gauges is thevariation in resistivity of the material that constitutes the straingauge grid. As well as this effect one must also consider the elongationof the conductors of the strain gauge. The dilating of the material onwhich the strain gauge is applied must also be considered.

The deformation applied on the grid is therefore the result of thedifference between the deformation of the material on which the grid isstuck and that which it would have in the absence of external actions.This deformation thus causes a variation in the internal resistance ofthe strain gauge; such a variation in resistance is proportional to theapparent deformation due to the effect of heat.

It is important to remember that the temperature, as well as the“interfering” effect just discussed, also has a “modifying” effect sinceit changes the value of the piezoresistive component.

The Applicant has foreseen two ways of compensating the effects oftemperature on the strain gauges.

A first way consists of using, as well as the strain gauge for measuringthe deformations (that we shall call “active gauge” for the sake ofsimplicity), a strain gauge identical to the first (that we shall call“dummy gauge”) but connected onto an adjacent side of a WheatstoneBridge (so that the deformations measured by the two strain gauges aresubtracted from each other) and mounted on a portion of the rim notsubjected to any deformation but subjected to the same temperature asthe active gauge.

Under this condition the thermal outputs of the two strain gauges areidentical and, through their mutual positioning on adjacent sides of themeasuring bridge, they are eliminated preserving just the actualdeformation component measured by the active gauge.

The second method for compensating the effects of temperature consistsof exploiting the self-compensating effect of strain gauges. This effectconsists of ensuring that the difference in thermal dilationcoefficients of material and strain gauge compensates for the variationin resistivity of the material that constitutes the grid of the straingauge. The self-compensation is, however, linked to a particularmaterial in the sense that each self-compensated strain gauge isnecessarily dedicated to a determined (and therefore single) material.

As an alternative to strain gauges, other types of sensors can also beused that are capable of detecting the local deformations of the rimwhen subjected to a load.

For example, in an alternative embodiment of the method of the presentinvention, instead of the strain gauges membrane sensors based upon SAWtechnology are used. These are membranes that vibrate in a different wayaccording to the voltage to which they are subjected. The membranes aremade to vibrate by an external energy source that applies a signal at adetermined frequency. The external source can be on the vehicle itself,so as to be easily supplied with power through the battery of thevehicle (since it is not on the rim there are thus no problems of powersupply). The ways of vibrating and thus the type of waves emitted by themembrane is a function of the state of excitation to which the membraneitself has been subjected. The system that emits the signals is alsocapable of reading the reply signals (reply waves) and of interpretingthem with the aim of translating such signals into deformations to whichthe membrane that has been made to vibrate is subjected. The positioningof the emitter/receiver system on the vehicle (for example on thesuspension) allows it to be easily supplied with power and allows thereply signal to be obtained directly on the vehicle (in a position wherethere is no rolling, differently than on the rim). Thetransmitter/receiver system also comprises the control unit thatprocesses the data so as to generate the command signal to be sent tothe wheel.

In a further alternative embodiment sensors can be used that are madewith the presence of materials that have variations in magneticproperties as the stresses to which they are subjected vary: thevariation in magnetic properties is usually measured through a variationin inductance.

EXAMPLE

The Applicant has carried out a series of experimental tests to verifythe reliability of the method of the present invention. The tests werecarried out by mounting three strain gauges like in the sample wheelillustrated in FIG. 2, so as to measure the deformation of the rim byflexing ε₁ in the plane perpendicular to the middle plane X-Z of thewheel, the circumferential deformation ε₂ of the rim and the deformationof the rim by flexing ε₃ in a plane parallel to the middle plane X-Z ofthe wheel.

In a first approximation it is hypothesised that the link between thedeformations of the rim and the forces at the hub generated by thewheel-road contact forces is linear, that is:

F=[A]·ε

where F is the vector of the forces at the hub, [A] is the correlationmatrix and ε is the vector of the deformations.

The vector of the forces was defined with the three force components Fx,Fy and Fz whereas that of the deformations was defined considering thedeformation value of the spoke on which the strain gauge is mounted whenit is at the footprint area, that is when such a spoke is at thewheel-road contact zone.

In this case the correlation matrix has a size of 3×3. Hereafter it isshown the calibration matrix identified in the following tests:

-   -   variation in vertical load from 2000 N to 6000 N at 50 km/h,        with zero drift, sliding and camber;    -   sinusoidal drift variation (magnitude 2°, frequency 0.25 Hz) at        50 Km/h with 3000 and 5000 N vertical load, with zero sliding        and camber;    -   sinusoidal variation in camber (magnitude 2°, frequency 0.25 Hz)        at 50 Km/h with 3000 and 5000 N vertical load, with zero sliding        and drift;    -   0-10-0% braking sliding at 50 km/h with 3000 and 5000 N vertical        load, with zero drift and camber.

The matrix obtained is the following:

$\begin{bmatrix}F_{z} \\F_{y} \\F_{x}\end{bmatrix} = {{10^{5} \cdot \begin{bmatrix}0.8564 & {- 2.1741} & {- 0.0232} \\{- 0.0788} & 1.2650 & 0.0381 \\{- 0.0088} & 0.0084 & {- 0.3449}\end{bmatrix}} \times \begin{bmatrix}{ɛ_{1}\left( {0{^\circ}} \right)} \\{ɛ_{2}\left( {0{^\circ}} \right)} \\{ɛ_{3}\left( {0{^\circ}} \right)}\end{bmatrix}}$

FIGS. 6-9 show a comparison between the forces that are applied(measured) and those that are determined (estimated) according to themethod of the present invention. It is clear that, with theaforementioned arrangement of the sensors, good results are obtained inthe estimation of:

-   -   lateral force Fy with zero sliding and camber;    -   longitudinal force Fz with zero drift and camber.

1-29. (canceled)
 30. A method for determining the force at the hub of awheel of a vehicle while traveling, said wheel comprising a rim and atleast one deformation sensor directly associated with said rim in atleast one predetermined position and arranged according to at least onepredetermined orientation, comprising the steps of: detecting at leastone deformation component of said rim during travel through said atleast one deformation sensor; and applying to said at least onedeformation component, during travel, a correlation parametercharacteristic of said rim, between the force at the hub and therelative deformation of said rim to determine at least one forcecomponent at the hub correlated with said at least one deformationcomponent of said rim.
 31. The method according to claim 30, whereinsaid correlation parameter is determined experimentally through thefollowing steps: a) providing a sample wheel having a rim which issubstantially the same as the rim of said wheel; and b) determining,through experimental tests on said sample wheel, at least onecorrelation coefficient between at least one force component acting uponsaid sample wheel at the hub along at least one predetermined directionand at least one respective deformation component of the rim of saidsample wheel.
 32. The method according to claim 31, wherein the step ofdetermining said at least one correlation coefficient comprises thesteps of: a1) setting said correlation coefficient at an initial value;a2) applying at least one test parameter to said sample wheel; a3)applying at least one force component acting upon the sample wheel atthe hub along said at least one predetermined direction; a4) detectingat least one deformation component of the rim of said sample wheelcaused by said at least one force component; a5) multiplying said atleast one correlation coefficient and said at least one deformationcomponent detected in step a4) to obtain at least one calculated forcecomponent; a6) comparing said at least one force component calculated instep a5) with said at least one force component applied in step a3); anda7) iteratively repeating steps a5) and a6), each time correcting thevalue of said at least one correlation coefficient until said at leastone force component calculated in step a5) assumes a value substantiallyequal, apart from a predetermined tolerance, to that of said at leastone force component applied in step a3).
 33. The method according toclaim 32, wherein the correction of said at least one correlationcoefficient takes place through an error minimization algorithm betweensaid at least one force component calculated in step a5) and said atleast one force component applied in step a3).
 34. The method accordingto claim 30, wherein said at least one deformation sensor is associatedwith said rim in at least one zone of maximum deformation of said rim ina determined direction.
 35. The method according to claim 30, whereinsaid at least one deformation sensor is a strain gauge.
 36. The methodaccording to claim 30, wherein said at least one deformation sensor is amembrane sensor.
 37. The method according to claim 30, wherein said atleast one deformation sensor is a sensor comprising materials that havevariations in magnetic properties as the stresses to which they aresubjected vary.
 38. The method according to claim 30, wherein said wheelcomprises at least three deformation sensors suitably positioned indifferent points of the rim of said wheel so as to detect during travelat least three local deformation components of the rim generated by theforce acting at the wheel-road contact zone.
 39. The method according toclaim 38, wherein said wheel comprises four sensors.
 40. The methodaccording to claim 30, further comprising the steps of: generating atleast one control signal representing said at least one force componentat the hub determined during travel; processing said at least onecontrol signal to determine data representing the dynamic conditions ofthe wheel during travel; and sending said data to a vehicle controlunit.
 41. The method according to claim 40, wherein the step ofprocessing said at least one control signal is carried out by aprocessing unit mounted on said wheel.
 42. The method according to claim40, wherein the step of processing said at least one control signal iscarried out by a processing unit mounted inside the vehicle.
 43. Themethod according to claim 40, wherein said control unit is mountedinside the vehicle.
 44. The method according to claim 40, wherein saidcontrol unit is mounted in a remote position with respect to saidvehicle.
 45. The method according to claim 40, further comprising thesteps of: processing said data in said vehicle control unit to generatea command signal representing the optimal dynamic conditions of saidwheel during travel; controlling the dynamic behavior of the vehicle bymodifying at least one travel parameter applied to said wheel, basedupon said command signal.
 46. The method according to claim 30, furthercomprising a power source mounted inside the wheel.
 47. The methodaccording to claim 30, further comprising a power source mounted insidethe vehicle.
 48. The method according to claim 47, further comprising aradio-frequency energy transmission system from said power source tosaid wheel.
 49. The method according to claim 30, further comprising asystem for compensating the effects of heat on said at least onedeformation sensor.
 50. A wheel for vehicles, comprising a rim and apneumatic tyre mounted on said rim, wherein said wheel comprises atleast one deformation sensor directly associated with said rim in atleast one predetermined position and arranged according to at least onepredetermined orientation.
 51. The wheel according to claim 50, whereinsaid at least one deformation sensor is associated with said rim at atleast one zone of maximum deformation of said rim in a determineddirection.
 52. The wheel according to claim 50, wherein said at leastone deformation sensor is a strain gauge.
 53. The wheel according toclaim 50, wherein said at least one deformation sensor is a membranesensor.
 54. The wheel according to claim 50, wherein said at least onedeformation sensor is a sensor comprising materials that have variationsin magnetic properties as the stresses to which they are subjected vary.55. The wheel according to claim 50, comprising at least threedeformation sensors suitably positioned in different points of the rimof said wheel so as to detect, during travel, at least three localdeformation components of the wheel generated by the force acting at thewheel-road contact zone.
 56. The wheel according to claim 55, comprisingfour sensors.
 57. The wheel according to claim 50, further comprising aprocessing unit of a control signal representing at least one forcecomponent at the hub.
 58. The wheel according to claim 50, furthercomprising a power source.